Re: [PATCH] dma-direct: avoid redundant memory sync for swiotlb

[Chao Gao]

On Tue, Apr 12, 2022 at 07:38:05PM +0800, Chao Gao wrote:
>When we looked into FIO performance with swiotlb enabled in VM, we found
>swiotlb_bounce() is always called one more time than expected for each DMA
>read request.
>
>It turns out that the bounce buffer is copied to original DMA buffer twice
>after the completion of a DMA request (one is done by in
>dma_direct_sync_single_for_cpu(), the other by swiotlb_tbl_unmap_single()).
>But the content in bounce buffer actually doesn't change between the two
>rounds of copy. So, one round of copy is redundant.
>
>Pass DMA_ATTR_SKIP_CPU_SYNC flag to swiotlb_tbl_unmap_single() to
>skip the memory copy in it.
>
>This fix increases FIO 64KB sequential read throughput in a guest with
>swiotlb=force by 5.6%.
>

Sorry. A fixes tag is missing:

Fixes: 55897af63091 ("dma-direct: merge swiotlb_dma_ops into the dma_direct code")

>Reported-by: Wang Zhaoyang1 <zhaoyang1.wang@xxxxxxxxx>
>Reported-by: Gao Liang <liang.gao@xxxxxxxxx>
>Signed-off-by: Chao Gao <chao.gao@xxxxxxxxx>
>Reviewed-by: Kevin Tian <kevin.tian@xxxxxxxxx>
>---
> kernel/dma/direct.h | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
>diff --git a/kernel/dma/direct.h b/kernel/dma/direct.h
>index 4632b0f4f72e..8a6cd53dbe8c 100644
>--- a/kernel/dma/direct.h
>+++ b/kernel/dma/direct.h
>@@ -114,6 +114,7 @@ static inline void dma_direct_unmap_page(struct device *dev, dma_addr_t addr,
> 		dma_direct_sync_single_for_cpu(dev, addr, size, dir);
> 
> 	if (unlikely(is_swiotlb_buffer(dev, phys)))
>-		swiotlb_tbl_unmap_single(dev, phys, size, dir, attrs);
>+		swiotlb_tbl_unmap_single(dev, phys, size, dir,
>+					 attrs | DMA_ATTR_SKIP_CPU_SYNC);
> }
> #endif /* _KERNEL_DMA_DIRECT_H */
>-- 
>2.25.1
>